Once installed :
- Run Tweak and click the "Janitor" button
- Click the checkbox next to "Old Kernel" under "System"
- Click the "Old Kernel" checkbox on the right, then the "Clean" button at the bottom right.
If you want to keep some of the old kernels, click the arrow next to "Old Kernel" checkbox, and they will be listed individually. You can then choose which to delete and which to keep.